This invention relates to a keyboard switch suitable for use with various kinds of equipment such as personal computers, word processors, etc., and particularly to a keyboard switch having key tops configured to be supported by means of pantograph mechanisms.
FIG. 1 illustrates an example of the prior art keyboard switch having such configuration wherein on a plate 1 constituting a keyboard substrate is disposed a membrane sheet 2 on which a housing 3 is carried.
As shown in details in FIG. 2, the membrane sheet 2 comprises a pair of laminated contact layers 2A and 2B spaced apart by a predetermined distance by a spacer 2C, and a contact pattern 2D and contact patterns 2E, 2F formed on the opposed surfaces of the contact layers, the contact pattern 2D and contact patterns 2E, 2F together comprising contacts. It is to be understood that upon pressure being applied to the contact layer 2A from above at the contact region, the contact pattern 2D and contact patterns 2E, 2F will be brought into contact with each other to establish electrically conductive continuity between the contacts 2E and 2F through the contact 2D to thereby provide a contact signal.
The housing 3 made of resin material has an opening 4 formed therethrough in opposition to the contact region to expose that surface of the membrane sheet 2 covering the contact region and its vicinity, and a dome-like rubber member 5 is disposed on the membrane sheet 2 within the region of the opening 4. The dome-like rubber member 5 comprises a vertically flexible skirt portion 5A and a push-button portion 5B connected with the top of the skirt portion 5A.
A pair of links 6 and 7 forming a pantograph mechanism is constructed as shown in FIGS. 3A and 3B. The link 6 comprises a pair of parallel link members which are interconnected at their intermediate portions by a central pressure-applying portion 6A for pressing on the rubber member 5. The parallel link members have stud shaft holes 6B formed on the opposite sides of the pressure-applying portion 6A for receiving respective stud shafts 7A extending inwardly from the opposed sides of a pair of link arms forming the other link 7. It is thus to be understood that the two links 6 and 7 are assembled together in an X-shaped form at their middle portions for relative pivotal movement by engaging the stud shafts 7A of the link 7 with the stud shaft holes 6B of the link 6.
More specifically, the first link 6 has a pair of stud shafts 6C extending inwardly toward each other from first ends of the link members. The link members are interconnected at their second ends by a cross-bar extending therebetween and terminating at its opposite outer ends in outwardly projecting stud shafts 6D. The second link 7 has a pair of stud shafts 7C extending oppositely outwardly from first ends of the link members. The link members of the second link are interconnected at their second ends by a cross-shaft 7B extending therebetween.
The stud shafts 7C are rotatably supported in corresponding journal bearings 17A formed in the bottom side of the key top 17 while the stud shafts 6D are rotatably supported in corresponding journal bearings 3A formed in the housing 3. On the other hand, the stud shafts 6C are slidably supported in corresponding slide bearings 17B formed in the bottom side of the key top 17 while the stud shafts 7B are slidably supported in corresponding slide bearings 3B formed in the housing 3. In this manner, the pressure-applying portion 6A of the link 6 is located in opposing contact with the top surface of the push-button portion 5B of the rubber member 5 whereby the links 6 and 7 are held in their erected position by the resilient restoring force of the rubber member 5, that is, the key top 17 is held in its top dead center.
Downward pressing operation on the key top 17 will move the links 6 and 7 comprising the pantograph mechanism toward the face of the housing 3 as the key top 17 moves parallel to the face of the housing 3. During this process, the pressure-applying portion 6A of the link 6 presses on and collapses the rubber member 5 downward to provide good tact feeling while at the same time the contact region of the membrane sheet 2 is pressed on by the push-button portion 5B whereby the ON-OFF operation of the contacts is effected.
It should be noted here that with an advance in downsizing and portability of equipment provided with the keyboard switch of the type discussed herein, there is an increasing demand for a keyboard switch both lighter in weight and lower in profile. Particularly, there is a need for a keyboard switch retaining the length of key stroke substantially equal to that of the conventional keyboard switch while having a further decreased thickness.
Nevertheless, in the prior art keyboard switch having the construction as described above, one of the factors for constructionally hindering reduction in thickness (vertical profile) is the thickness of the housing 3. Specifically, since the housing 3 is formed of resin, it is not permitted to make it too thin due to the moldability as well as from the point of view of the rigidity and thermal stability as a housing, resulting in the disadvantage of inhibiting the reduction in thickness.